Welcome to GLG101C Introduction to Geology Fall 2004 Professor James Tyburczy
Department of Geological Sciences
	Chapter 7 Notes

In addition to your textbook,

here is a site with some good pictures of weathering

Chapter 7. WEATHERING & EROSION - weathering and erosion are important parts of the Rock Cycle

Weathering - break up bedrock

• Physical weathering - fragmentation, break in to smaller pieces

• Chemical weathering - chemical alteration of the minerals

Erosion - processes that loosen soil and rock and move them downhill or downstream where they are deposited as sediment

Geological factors that control weathering

• Rock type and mineralogy - limestone easily weathered, quartz is resistant

• Climate - temperature and rainfall

• Effects of preexisting soil - retains moisture (water) and enhances acidity

• Time

Chemical weathering - decomposition of minerals by chemical reaction because the minerals are not in chemical equilibrium at the Earth's surface. Chemical weathering of Feldspar is an example to illustrate major points

Feldspar + H2O + acid = clay (solid) + dissolved SiO2 + Dissolved potassium ions (K+) + other dissolved ions...

Features of chemical weathering

• Requires water - water is incorporated into the mineral structure of many weathering minerals (example feldspar weathering to clay)

• Speeded up by acid. Carbonic acid (H2CO3) is most important - formed from H2O +CO2. Man-caused acids (nitrogen oxides and sulfur oxides into the air from fossil fuels) increasing in importance today

• Solid products of chemical weathering-> clay minerals, quartz, iron oxides, aluminum oxide

• Dissolved products -> ions in solution (for example K+, Na+, Ca+2, etc.), plus dissolved silica (SiO2). Acid rain speeds up dissolution of monuments and statues.

• Chemical weathering of other minerals - iron silicates, aluminum in silicates, carbonates

  Iron silicates (weather to reddish iron oxides)

  Aluminum in silicate minerals (weathers to form aluminum oxides - lateritic soils and ultimately bauxite (Al ore))

  Carbonates (weather completely to dissolved Ca+2 ions and dissolved bicarbonate ions)

• Chemical stability and relative weathering rates - compare to crystallization from magma - minerals stable at high T in magma (for example, olivine & pyroxene) are least stable with respect to chemical weathering at the low temperature conditions of the Earth's surface, and vice versa (quartz as an example).

• When rock weathers, the climate system and the lithosphere interact. Atmospheric CO2 influences climate and weathering. If CO2 is elevated in the atmosphere, the atmosphere warms, causing more weathering, which removes CO2 from the atmosphere, which causes cooling of hte atmosphere, which reduces the weathering rates, which causes CO2 levels in the atmosphere to increase, causing warming.... (see Fig 7.6)

Physical or mechanical weathering - physical disintegration of rock

• Freeze/thaw - frost wedging, frost heaving, because water expands when it freezes to form ice.

• Natural zones of weakness in rocks - sedimentary bedding, metamorphic foliation

• Pressure release, opening of joints - exfoliation of massive igneous rocks

• Growing of crystals, especially salt crystals, in cracks (arid regions)

• Thermal expansion - daily hot and cold cycles - active on the moon?

• Plants and animals - biological activity, roots, bacteria

• Other forces - rivers, glaciers, waves. Grinding, abrasion

Soils - layer of weathered, unconsolidated material on top of rock

Residual soil compared to Transported soil

Controls on soil formation

• Climate (most important)

• Parent material

• Time

• Plants & animals

• Presence of soil (positive feedback)

• Particle size

Soil Profile -

• Humus layer

• A Horizon - zone of leaching

• B- horizon - zone of accumulation

• C horizon - zone of slightly altered bedrock

Climate affects type of soil

• Humid, temperate climate - moderate leaching - pedalfer

• Tropical - intense leaching - poor soil for growing - laterite (form ores which can be mined for aluminum)

• Desert - water evaporates, calcite precipitates - pedocal

  Caliche - rock fragments cemented by calcite

Paleosols - Ancient soils preserved in the rock record - studied to get ideas of past climate - Painted Desert of northern Arizona (Chinle formation)

Humans as weathering agent

• Acid rain from CO2, SO2, NO2 into air - chemical weathering

• Construction of roads and buildings - physical weathering

• Promote much faster weathering than natural processes alone